1. Field of the Invention
This invention relates to communication systems and more particularly to apparatus for use in establishing and maintaining communication with a controlling module which transmits data and control commands serially by bit over a serial transmission medium, and with one or more I/O modules over a demand/response I/O interface.
2. Description of the Prior Art
Serial communication media have been used for many years to control the transfer of data from a number of I/O modules, such as interactive terminals, and remote data processing systems. Early transmission media utilized serial telegraph, start-stop formats or synchronous serial data streams. More recently serial data transmission loops have been used wherein time slots are assigned to I/O terminals and messages to and from the terminals are transmitted in the assigned time slots.
In any of these techniques, there is a necessity to match the interactive, demand/response operation of a number of I/O terminals with the serial bit stream synchronization of the communication medium.
In addition to the problems which arise with respect to synchronizing different data formats, modern day technology requirements also impose constraints. In modern technology, circuit elements are interconnected and are inseparably associated with a continuous supporting material upon which the circuits are fabricated. The integrated circuit becomes part of a larger communication or data processing system. One of the problems associated with this type of circuitry is that there are a limited number of input/output access points or pins on the integrated circuit to provide external connections. Therefore, the need has arisen for a compact and simple data flow which involves the use of a minimum number of input/output lines. In order to reduce the number of integrated circuits necessary to fabricate an entire system, there is a need for as little logic as possible on the support material or substrate. The trend is therefore toward using a microprocessor wherein many of the functions formerly performed by hardware logic are performed by a sequence of micro-instructions. Therefore, it is important that a small instruction set be utilized.
A problem unique to the synchronization needs of adapting a serial medium with a demand/response terminal interface is that of sufficient interlock between the serial medium and the microprocessor and between the microprocessor and the terminal interface.
While the prior art devices have utilized microprocessors for controlling input/output operations they have not satisfactorily resolved the problems of adapting modern technology to the special problems associated with the synchronization of serial communication media with demand response input/output terminals.